Polymer resins, such as polyethylene terephthalate (PET), are widely used in the packaging industry. PET is a linear, thermoplastic polyester resin. The advantages of PET include toughness, clarity, good barrier properties, light weight, design flexibility, chemical resistance, and good shelf-life performance. Furthermore, PET is environmentally friendly because it can often be recycled. These characteristics of PET make it a popular material in the manufacturing of containers, for example, beverage bottles.
There are a variety of production methodologies to produce PET containers. For example, injection stretch blow molding is commonly used to make PET bottles. Of the various methodologies, one-piece PET containers having an integrated handle are commonly formed using extrusion blow molding (EBM). The EBM process includes extruding a polymer resin in a softened state through an annular die to form a molten hollow tube or parison. The molten parison is placed in a hollow blow mold having a cavity corresponding to the desired shape of the container being formed. Air is injected to inflate the parison against the interior walls of the blow mold. Upon contact with the walls, the parison cools rapidly and assumes the shape of the mold.
PET manufacturers have developed different grades or versions of PET that are more suitable for use in EBM methods. Such extrusion grade PET or “EPET” has a higher molecular weight that standard PET, and has an inherent viscosity of 1.0 dl/g or greater as measured by solution viscosity. Importantly, the molding forces associated with forming EPET containers are higher than the molding forces developed during the forming of PET containers.
One type of rapid EBM method used to from EPET containers uses a blow mold apparatus that has the capacity of forming approximately 100 containers per minute. This type of blow mold machine may be configured with a rotating vertical wheel such that the wheel is configured with circumferentially spaced mold halves. The mold halves each capture a vertical, continuously growing parison at an extrusion station. In one example blow mold apparatus the flow head extruding the parison moves up and away from the mold halves after the mold halves close to capture the parison. The parison is severed adjacent to the top of the mold halves, the mold halves then move away from the extrusion station, and a top blow pin is moved into the end of the captured parison at the top of the mold halves to seal the mold cavity and blow the parison. The flow head and dependent parison are then lowered back into the initial position so that the new parison is in position to be captured by the next pair of mold halves.
The blown parison cools as the wheel and mold halves rotate. At the appropriate mold ejection station, the mold halves open and the finished container is ejected from between the mold halves. In normal operation, the rotary wheel apparatus is capable of producing approximately 110 containers per minute using a 22 mold machine and a rotation rate of 5 RPM.
EBM processes, and especially those forming EPET containers, often develop very high mold clamp forces. Because the mold clamp forces are transmitted through the system linkages, any weak element or component in the linkage may result in stress failures or deformation in the element. One element in the system linkage that is subjected to such high forces is a cam follower slide. Incorporating an element within the system linkage to absorb certain excess forces that may be transmitting through the linkage system and the two mold halves, in the fashion of a shock absorber, would reduce certain deformations or stress failures in the linkage system. Such deformations could cause increased system fatigue, mold misalignment, or even system failure.
Accordingly, there is a need to incorporate a means to absorb excess forces generated during the EBM process, and thereby reduce linkage deformations or stress failures, and increase system reliability and consistent container results. Such an improvement has not been previously incorporated into such EBM machines, including those forming EPET containers. The inventive cam follower element described below, incorporating spring means, is such an improved system linkage element designed to absorb excess system linkage forces.